1. Field of the Invention
This invention relates to novel fiber-forming polyhydrazides and to useful yarns made therefrom.
2. Description of the Prior Art
U.S. Pat. No. 3,748,298 describes continuous filament yarns prepared from polyhydrazides consisting essentially of recurring structural units of the formulas (I) and (II) ##SPC1##
in a mole ratio of from 9:1 to 1:9. Tire cord may be made from these yarns in a conventional manner, wherein two or more of the yarns are each twisted a predetermined number of turns per inch (tpi) in one direction (e.g., a clockwise direction) and then plied a substantially corresponding number of tpi in the opposite direction to provide a balanced and highly twisted structure.
The twist in a cord structure gives the cord the ability to absorb compression forces and withstand fatigue. It is well known that the fatigue resistance of a cord increases with increasing twist in the cord structure to a maximum value and thereafter decreases with any further increase in twist. For a given type of yarn such as nylon, polyester or polyhydrazide yarn the amount of twist required to reach this maximum value depends on the size or denier of the yarns. In order to express this amount of twist in yarns of different denier, it is common to use a quantitative index known as a twist multiplier (TM). The twist multiplier for a given type of yarn is determined by the following formula: ##EQU1## WHERE: T is twist in yarn, expressed as turns per inch (tpi); D is total denier of the untwisted yarn in the cord; and Sp. Gr. is specific gravity of the yarn. The derivation of the above formula is described in U.S. Pat. No. 3,233,648.
It is also well known that the strength of a cord in terms of its tenacity decreases with increasing twist in the cord structure. Therefore, tire cords normally will contain the minimum amount of twist necessary to provide a cord of acceptable fatigue resistance. Thus, for a given tire yarn there is an optimum twist multiplier (TM.sub.o) which will provide a cord having an optimum balance of properties with regard to fatigue resistance and tenacity for its particular end use in a tire. In this way the loss of tenacity resulting from twisting the yarns will be minimized. Naturally, the optimum twist multiplier (TM.sub.o) has a lower value and involves less twist than the twist multiplier (TM) that is used to provide cord with maximum fatigue resistance.
It has been found that when yarns of the polyhydrazides described in U.S. Pat. No. 3,748,248 are plied (i.e., twisted) into cord using the OTM of 6.89, the tenacity of the resulting cord is less than half that of any of the untwisted yarns. The twist efficiency of yarns in terms of percent tenacity retained (T.sub.R) upon yarn-to-cord conversion is an important characteristic of yarns used in making cords and may be represented by the following equation: ##EQU2##
It is an object of the present invention to provide novel polyhydrazides and yarns prepared therefrom having T.sub.R values greater than about 55%.